A nitride semiconductor including nitrogen (N) as a Group V element is a prime candidate for a material to make a short-wave light-emitting device because its bandgap is sufficiently wide. Among other things, gallium nitride-based compound semiconductors (which will be referred to herein as “GaN-based semiconductors”) have been researched and developed particularly extensively. As a result, blue light-emitting diodes (LEDs), green LEDs, and semiconductor laser diodes made of GaN-based semiconductors have already been used in actual products.
A gallium nitride-based semiconductor has a wurtzite crystal structure. FIG. 1 schematically illustrates a unit cell of GaN. In an AlaGabIncN (where 0≦a, b, c≦1 and a+b+c=1) semiconductor crystal, some of the Ga atoms shown in FIG. 1 may be replaced with Al and/or In atoms.
FIG. 2 shows four primitive vectors a1, a2, a3 and c, which are generally used to represent planes of a wurtzite crystal structure with four indices (i.e., hexagonal indices). The primitive vector c runs in the [0001] direction, which is called a “c-axis”. A plane that intersects with the c-axis at right angles is called either a “c-plane” or a “(0001) plane”. It should be noted that the “c-axis” and the “c-plane” are sometimes referred to as “C-axis” and “C-plane”, respectively.
The wurtzite crystal structure has other typical crystallographic plane orientations than the c-plane, as shown in FIG. 3. FIG. 3(a) shows a (0001) plane. FIG. 3(b) shows a (10-10) plane. FIG. 3(c) shows a (11-20) plane. FIG. 3(d) shows a (10-12) plane. As used herein, “-” attached on the left-hand side of a Miller-Bravais index in the parentheses means a “bar” (a negative direction index). The (0001) plane, the (10-10) plane, the (11-20) plane, and the (10-12) plane are the c-plane, the m-plane, the a-plane, and the r-plane, respectively. The m-plane and the a-plane are “non-polar planes” that are parallel to the c-axis, and the r-plane is a “semi-polar plane”. Note that the “m-plane” is a generic term that collectively refers to a family of planes including (10-10), (−1010), (1-100), (−1100), (01-10) and (0-110) planes.
For years, a light-emitting device in which a gallium nitride-based compound semiconductor is used is fabricated by means of “c-plane growth”. As used herein, the “X-plane growth” means epitaxial growth that is produced perpendicularly to the X plane (where X=c, m, a, or r) of a hexagonal wurtzite structure. As for the X-plane growth, the X plane will be sometimes referred to herein as a “growing plane”. Furthermore, a layer of semiconductor crystals that have been formed as a result of the X-plane growth will be sometimes referred to herein as an “X-plane semiconductor layer”.
When a light-emitting device is fabricated using a semiconductor multilayer structure formed by means of the c-plane growth, spontaneous polarization occurs in the −c direction due to a shift in the c-axis direction between the positions of a Ga atom and a N atom on the c-plane. On the other hand, in an InGaN quantum well layer used in a light-emitting layer, a piezoelectric polarization occurs in the +c direction due to strain, and a quantum confinement Stark effect of carriers occurs. Therefore, it is called a “polar plane”. This effect reduces the probability of radiative recombination of carriers in the emission section and accordingly reduces the internal quantum yield. In the case of a semiconductor laser device, the threshold current increases. In the case of an LED, an increase in power consumption or a decrease in emission efficiency is caused. Meanwhile, as the density of injected carriers increases, screening of the piezoelectric field occurs, and a variation in emission wavelength also occurs.
In view of such circumstances, in recent years, intensive research has been carried out on growth of a gallium nitride-based compound semiconductor on a non-polar plane, such as m-plane and a-plane, and a semi-polar plane, such as r-plane. If a non-polar plane is available as the growing plane, no polarization occurs in the layer thickness direction (crystal growth direction) of the emission section. Therefore, the quantum confinement Stark effect does not occur. Thus, a light-emitting device which potentially has high efficiency can be fabricated. Even when the growing plane is a semi-polar plane, the influence of the quantum confinement Stark effect can be greatly reduced.
Patent Document 1 discloses the method of obtaining the optimum growth conditions for an InxGa1-xN (0<x<1) layer based on the mole fraction of a source gas containing In (In supply mole fraction) and the diagram of the characteristic of the temperature employed for crystal growth (growth temperature) and the emission wavelength. A drawing of Patent Document 1 shows a graph where the abscissa axis represents the mole fraction of the In source gas with respect to the Group III source gas, and the ordinate axis represents the emission wavelength. This graph shows a characteristic curve in which the growth temperature is considered.